The present invention relates to a process for producing a relief structure on a semiconductor material support. This process more particularly makes it possible to produce in advantageous manner devices for binary optics on a semiconductor material support, more especially silicon.
The basic principles of binary optics are simple and apply both to optics in reflection and optics in refraction. These principles are defined in the article entitled "Binary optics" by W. B. VELDKAMP and T. J. McHUGH, published in Scientific American, May 1992.
One of the simplest structures which can be produced in binary optics is the prism, which is in the form of a series of staircase microsteps cut into the surface of the material. When the wave front reaches the surface, the wave is subdivided into secondary wave fronts in accordance with the Huygens-Fresnel principle. Each secondary wave front is delayed in proportion to the height of the step at this point, i.e. the traversed optical path supplement. When the wave fronts interfere with one another, they produce a new wave front propagating with a certain angle with respect to the direction of the incident wave front.
In the binary optics approach, it is sufficient to have a material thickness of approximately 1 .mu.m (corresponding to the wave length .lambda. of an incident light beam) to obtain the deflection effect, whereas in the conventional approach it would have been necessary to have a piece of glass with a thickness of several millimeters.
Thus, the production of binary optics devices makes it necessary to be able to locally produce on the surface of a material areas in overthickness or underthickness (the overthickness or underthickness having to be precisely obtained, because they correspond to a fraction of the use wavelength of the device). The location of these areas is a function of the optical function to be produced. In the same optics, there can be several ranges of overthicknesses or underthicknesses, e.g. corresponding to step differences of .lambda./8, .lambda./4, 3.lambda./8, .lambda./2, etc.
Conventionally, in order to obtain such a binary optics device, use is made of thin film deposition methods, when is wished to obtain a relief structure, associated with lithography and etching methods for producing overthickness areas. When it is wished to have a hollow structure, use is made of lithography methods for location and etching (e.g. in dry form).
These prior art production processes suffer from disadvantages with respect to the control and reproducibility of the thicknesses of the areas and also relative to the complexity of the operations necessary for producing several thickness ranges.
With regards to the control and reproducibility of the thicknesses, the result obtained is linked with the reaction kinetics and the reaction application time. In general, the parameters of the process are regulated and it is applied for a predetermined period of time. In general there is no indicator, during the application of the process, of the current thickness or depth. There can be a very considerable uncertainty with regards to the result.
For producing several thickness ranges, it has been found that when the process is used for obtaining a thickness range in a given area, the other areas must be protected against the application of the process. This leads to complexity with regards to the lithography, as well as boundary problems between the areas due to imperfect alignments.
In addition, FR-A-2 681 472 discloses a process for the production of thin semiconductor material films involving three stages. In a first stage, creation takes place by ion implantation of a layer of gas microbubbles under one face of a semiconductor material wafer. In a second stage, the wafer face is joined to a stiffener. In a third stage the assembly constituted by the wafer and the stiffener is subject to a temperature adequate for creating, by the crystal-line rearrangement effect in the wafer and pressure in the microbubbles, a cleaving of the wafer level with the layer of microbubbles, which gives a thin film adhering to the stiffener.
The present invention proposes a novel process for producing a relief structure on a semiconductor material support using a procedure derived from the teaching of FR-A-2 681 472. Thus, the novel process makes it possible to obviate the disadvantages of the prior art production processes.